FB Alpha Blend with PSET

[rdc]

I came across this http://www.gamedev.net/reference/articles/article874.asp on lens flare (which I was thinking of using) but I discovered a section on alpha blending. Since the FB alpha parm only works with PUT I converted the C code to FB so I could use it with PSET (or buffer). Here is the code:

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'Based on article: http://www.gamedev.net/reference/articles/article874.asp
option explicit

const sw = 640
const sh = 490
const centerx = 640/2
const centery = 480/2

dim as integer alpha, invalpha, x, y
dim as integer sr, sg, sb, cr, cg, cb, r, g, b

screenres sw, sh, 32

'Alpha and Inverse alpha
alpha = 100
invalpha = 255 - alpha
'background color
sr = 255
sg = 0
sb = 0
'Overlay color
cr = 255
cg = 255
cb = 0
'Draw a box
line (centerx - 120, centery - 120)- (centerx + 120, centery + 120), RGB(sr, sg, sb), BF
'Overlay a yellow box on red box
for x = centerx - 100 to centerx + 100
  for y = centery - 100 to centery + 100
    'calc the alpha values
    r = ((cr * alpha) + (sr * invalpha)) shr 8
    g = ((cg * alpha) + (sg * invalpha)) shr 8
    b = ((cb * alpha) + (sb * invalpha)) shr 8
    pset(x, y), RGB(r, g, b)
  next
next

sleep


While it is not particularly speedy, it does seem to work.

[Shockwave]

Thanks for sharing it Rick

[rdc]

No problem, it is quite a nice article. I am going to try and give that lens flare example a try. I think if someone new some asm, you could probably speed up the calculations for this quite a bit, although it may be acceptable using a ptc buffer rather than pset, which is quite slow. Ar any rate, I am going to try and use it for my demo and see how it works out.

[Shockwave]

It would definately work quite well using ptc I've used alpha myself in quite a few things and been pleased with the results.